The soil that makes Napa Valley grapes so special also makes the region vulnerable to earthquakes

Early Sunday morning, a magnitude-6.0 earthquake rumbled through Northern California. It was the largest quake to hit the Bay Area since the 1989 Loma Prieta earthquake, a magnitude-6.9 temblor that collapsed the Bay Bridge. With an epicenter just nine miles south of the town of Napa, the quake left dozens injured and damaged historic buildings throughout the Napa Valley.

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All told, the region is thought to have sustained upwards of $1 billion in damages, and one sector has seen some especially tragic loses: Napa's wine industry, which had just begun harvesting its 2014 crop. In an interview with the Associated Press, Tom Montgomery of B.R. Cohn Winery in Glen Ellen, California, estimated that as much as 50 percent of the winery's product was destroyed in the quake. "It's not just good wine we lost," Montgomery told the AP. "It's our best wine."

In an average year, Napa's wine industry generates $50 billion. The nonprofit group Napa Valley Vintners says that it's too early to estimate the amount of damage the earthquake caused, though their website states that it "is not expected to have a significant impact on Napa Valley wine inventory in general." And geologically speaking, earthquakes are a major reason Napa has become synonymous with wine.

"This is the kind of earthquake that created the Napa Valley, or at least the final morphology of the valley now," says Ken Verosub, professor of earth and planetary sciences at the University of California, Davis. "There's nothing here that's a big surprise."

The Napa Valley sits at the northern end of the San Francisco Bay, between the Vaca Mountains to the east and the Mayacamas Mountains to the west. The entire area rests on what is known as a transform fault zone: an area where two of Earth's tectonic plates slide past each other. In the case of the Bay Area, the sliding of the Pacific plate past the North American plate drives activity along the famous San Andreas fault zone. This major plate-boundary fault visibly stretches for 600 miles lengthwise through California.

But 40 million years ago, another crucial plate helped shape the Napa Valley of today. Back then the oceanic Farallon plate was subducting, or diving under, the North American plate. During subduction, some material is scraped from the plates and deposited on Earth's surface rather than sinking into the planet. As the Farallon plate moved under what is now California, it deposited a mix of material, so that today the western half of Northern California boasts a panoply of mineral riches, including blocks of limestone and sedimentary rock as well as fragments of the ancient sea floor.

At the same time, the Farallon plate was pulling away from the Pacific plate to the west, creating a "spreading center" where hot rock oozes up to fill the gap. Around 30 million years ago, this spreading center began to dive under the North American plate, and the San Andreas fault was born. Heat from the spreading center then triggered volcanic activity along the southern and northern boundaries of the transform fault. Volcanic rocks up to eight million years old have been found in the eastern part of the Napa Valley, says Verosub.

The valley itself formed as a result of a fault step-over—part of the network of complex fractures that branch off the main San Andreas fault line. Within a step-over, a particular fault jumps over an area of land but then continues in the same direction. Think of it like drawing a line on a piece of paper, stopping, moving your pencil down a few inches and continuing the same line. The area between a step-over is put under an immense amount of geological tension, which in some cases can cause the land to sink down, effectively creating a valley.

Fault activity, as well as erosion via wind and rain, continued to break apart the many types of rocks around the valley, depositing their riches on the valley floor. The end result is the Napa Valley's spectacular diversity of soil: over 100 variations, or equal to half of the world's soil orders. In wine growing, soil diversity is extremely advantageous, allowing numerous grape varieties to grow in a relatively small area. In the southern part of the Napa Valley, for instance, the calcium-rich soil favors pinot noir grapes. In the north, more volcanic soils help cabernet grapes thrive.

Diverse soil isn't the only remnant of the valley's tectonic past. "The Napa Valley has a large climatic gradient due to the geomorphology created by the tectonics," says Verosub. Hills and knolls formed by megaslides from the Vaca Mountains millions of years ago influence the climate of the valley floor. All told, the Napa Valley is home to 14 distinct American Viticultural Areas, each of which is completely unique due to its combined soil and climate.

With continued activity along the fault, Napa remains vulnerable to earthquakes like the one on Sunday. Moreover, the high amounts of sediment on the valley floor means the region really feels any shaking caused by tectonic movement. "[The sediment] may be great for grapes, but when there’s an earthquake anywhere in proximity, you get amplification," says Susan Hough, a seismologist at the United States Geological Survey in Pasadena, California. Still, any damages sustained during the earthquake may simply be the price Napa vintners pay for growing in such a geologically attractive part of the world.

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